The present invention relates in general to a process for preparing normally crystalline vinylidene halide polymers. More specifically, it relates to an improved process for preparing these polymers by suspension polymerization whereby spheroidal globules of polymer can be obtained in a generally uniform and predetermined small size.
For purposes of this application, the term "normally crystalline vinylidene halide polymers" is meant to include the homopolymers and copolymers of vinylidene halides, such as vinylidene chloride, vinylidene bromide, and the like and interpolymers of a vinylidene halide with other monoethylenically unsaturated monomers, such as vinyl chloride, vinyl acetate, acrylonitrile, butyl acrylate, butadiene, and the like, wherein the vinylidene halide is present in predominant amount and preferably constitutes at least 50 percent of the weight of the monomeric material from which the copolymer is prepared. The normally crystalline vinylidene halide polymers have long been recognized as valuable materials for preparing films, foils, fibers, filaments and molded articles. Processes for preparing such polymers in aqueous emulsion or in non-emulsified aqueous suspension have been known for a long time. For several reasons the suspension techniques are preferred. Emulsion polymerization gives particles which frequency are so finely divided and of such low bulk density as to be inconvenient for use in molding or extrusion. Further, the emulsions require an extra coagulation step which is both costly and time consuming. Generally, in the suspension processes a water-soluble or water-dispersible granulating agent is used to cause the polymer to be in the form of spheres which are small enough to be easily handled yet large enough to be free flowing. When no granulating agent is used, the polymerizing particles agglomerate or coalesce into large, hard lumps of polymer which must be ground before use.
In U.S. Pat. No. 2,932,629, to R. M. Wiley, there is disclosed a process for preparing solid polymeric bodies in the form of spheroidal globules having substantially uniform size by forming a suspension of droplets of a polymerizable liquid, such as vinylidene chloride, in an aqueous non-solvent medium by vigorous agitation, and thereafter subjecting the suspension to a condition of quiescence whereby a limited coalescence of the dispersed polymerizable liquid occurs with the formation of a dispersion of a lesser number of larger, stable, uniform-sized droplets, the size of which depends in a predictable manner on the composition of the aqueous suspending medium, and subsequently polymerizing the droplets with a peroxy catalyst. That reference also teaches that in some instances, e.g., when polymerizing vinylidene chloride, it is beneficial to treat the uniform droplet suspension prior to polymerization so as to render the suspension stable against agglomeration of the oil droplets. The stabilization is accomplished by gently admixing with the uniform droplet suspension an agent capable of greatly increasing the viscosity of the aqueous liquid. Suitable agents are any water-soluble or water-dispersible thickening agents, such as sulfonated polystyrenes, hydrophilic clays, digested starch, natural gums, carboxy substituted cellulose ethers, and the like.
The process disclosed in U.S. Pat. No. 2,932,629 is not suitable, however, for preparing normally crystalline vinylidene halide polymers in the form of spheroidal globules having a generally uniform and small, e.g., about 40 microns or less, size. Furthermore, the suspensions of that process, when treated with thickening agents to prevent agglomeration of the oil droplets, are undesirably very viscous and difficult to handle without modification of conventional suspension polymerization equipment.
Accordingly, it would be desirable to have a process for preparing normally crystalline vinylidene halide polymers in the form of spheroidal globules which would not require thickening of the aqueous suspension in order to prevent the monomer oil droplets from agglomerating during polymerization. Furthermore, it would be desirable to have a process for preparing such polymers in the form of spherodial globules having a generally uniform size, especially a generally uniform, small size, e.g., about 40 microns or less.